The present invention relates to a method for stabilizing sources of seismic energy in connection with marine seismic surveys where seismic sources are towed behind a vessel and are suspended below a buoyancy unit from lines the lengths of which may be adapted by actuation of a driving member.
The invention also relates to a device for utilization with such a method.
In marine seismic surveys seismic pulses are emitted from one or more sources of energy which are towed in the water, suspended from the buoyancy means. Seismic surveys have the object of providing data which describe the subterranean stratifications with the best possible accuracy. For this purpose pulses with adapted frequencies are emitted from the sources of seismic energy, so that the signal penetrates the crust of the earth and is partly reflected by various strata. The corresponding reflections are detected by the aid of a seismic receiver cable, a so called streamer, which may have a length of several thousand meters.
Sources of seismic energy commonly constitute so called air, gas or water guns. In surveying shallow waters sources which discharge an electrical plug are also used.
In such surveys, with a series of seismic guns being towed, one behind the other in the water, and where the emitted signals are to be reflected by various geological strata which are distributed across large areas, a great amount of background noise will appear, and a series of inaccuracies of measurements will result both on the emitter side and on the receiver side. Previous efforts were mainly directed to avoid background noise and to achieve as accurate and clear signals as possible. In this connection inaccuracies due to movement of the water, i.e. wave motion which will especially cause depth displacement of the positions of signal emitters, were ignored. The only connection in which the presence of such wave motion was taken into consideration was the design of the towed body from which the seismic guns are suspended, so as to make such a body glide as smoothly as possible in the sea and, consequently, move in a linear manner. The mentioned principles and associated problems are described in an earlier Norwegian Patent of the applicant's No. 150 016. The same approach to the problem is also found in Norwegian Patent No. 148 762, and in Norwegian Patent No. 16 478.
None of the mentioned concepts, however, provided a satisfactory solution of the problem of inaccurate measurements in heavy sea, since even with the compensation of wave motion achieved by the previously known systems there will still be differences in height between the locations of separate air guns in heavy sea.
With common use of air guns which are placed in groups it is, however, very important that the guns are kept as much as possible at the same level during firing. In case of, e.g. air guns which are mutually adapted to be considered a point firing source (as disclosed in Norwegian Patent No. 147 655), motion during firing operations, caused by wave action or another corresponding noise interference, will cause the emitted impulse to change its character from one shot to the next.